ASTM A312 vs ASTM A358 Stainless Steel Pipe: Scope and Selection

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

Introduction

The question of ASTM A312 vs A358 comes up whenever a buyer or engineer is choosing between two of the most important stainless steel pipe standards. The confusion is understandable: both cover austenitic stainless steel pipe, and both appear on quotations for similar applications. But they are not interchangeable — they govern different product forms and manufacturing methods, with different testing and inspection expectations.

This guide compares the two standards on scope, manufacturing, and quality acceptance, and explains when each is the correct choice. The goal is not to rank one standard as "better" — it is to match the right standard to the right product and service requirement.

1. ASTM A312 vs ASTM A358: Quick Comparison

2. What ASTM A312 Covers

ASTM A312/A312M covers austenitic stainless steel pipe supplied in three product forms:

• Seamless — formed without a longitudinal weld seam.
• Welded — formed from strip or plate and joined by a longitudinal weld.
• Heavily cold worked — cold worked to achieve elevated strength.

It is not correct to call A312 a "seamless-only" standard. It covers both seamless and welded pipe, and the manufacturing method must be stated on the purchase order because the two are not interchangeable by default.

3. What ASTM A358 Covers

ASTM A358/A358M covers electric-fusion-welded (EFW) austenitic chromium-nickel stainless steel pipe for high-temperature service and general applications. The defining feature is the manufacturing method: the pipe is produced by electric fusion welding, where an electric arc fuses the joint.

A358 is not simply a "generic welded pipe" standard. It is specifically for electric-fusion-welded pipe, and it uses a Class system to define the weld configuration and the required radiographic examination. This makes it particularly relevant for larger-diameter, fabricated welded pipe where weld quality and inspection level must be explicitly controlled.

4. Manufacturing: A312 vs A358

The manufacturing distinction is the heart of the A312 vs A358 comparison:

• A312 welded pipe is produced by welding a longitudinal seam; the standard also includes seamless and heavily cold worked forms.
• A358 pipe is electric-fusion-welded — the joint is made by electric arc fusion. This is a different welding concept from other welded-pipe methods.

It is important not to conflate different welded-pipe terms. ERW (electric resistance welding) and EFW (electric fusion welding) are different processes, and SAW (submerged arc welding) is a specific fusion welding method. A358's "electric-fusion-welded" description should not be assumed to mean ERW or a single specific arc process — the governing requirement is the A358 standard and its Class definitions.

5. A358 Class 1–5 Explained

A358 uses a Class system to define two things: the weld configuration (single-weld or double-weld joint) and the required radiographic examination of the weld. The Classes are not a simple "quality ranking" — they are different manufacturing and inspection requirements for different service needs.

The Classes are generally understood to differ as follows, with the exact requirements confirmed against the current standard edition:

• Classes based on double-welded joints versus single-welded joints.
• Within each weld configuration, the Classes specify the extent of radiographic examination — full (100%) radiography, spot radiography, or a reduced/as-agreed examination level.

The practical meaning for a buyer: when ordering A358 pipe, the Class must be stated, because it controls both how the weld is made and how thoroughly it is inspected. A Class with full radiography is selected for higher-integrity or code-governed service; a Class with spot or reduced examination is selected where the application does not require full radiographic coverage. The Class is a specification choice, not a material quality grade — the base metal grade (e.g., 316L) is specified separately.

6. Grades, Sizes and Dimensions

Both standards cover austenitic stainless steel grades, with 304/304L and 316/316L among the most commonly specified. A grade's presence in one standard should not be assumed to mean it is covered by the other without checking — confirm the specific grade and UNS against the applicable standard.

Neither A312 nor A358 is a complete dimensional standard. Pipe is ordered by NPS, outside diameter, wall thickness or schedule, and length, with actual dimensions and tolerances confirmed against the applicable general-requirements standard and size tables. Do not assume a universal "±mm" tolerance, and do not treat schedule as a material grade — it is a wall-thickness designation that varies with NPS.

7. Testing, Inspection and Radiography

Testing requirements must be read carefully against each standard, because the applicability varies:

• Chemical analysis — to confirm grade composition.
• Tensile requirements — for strength and elongation.
• Hydrostatic test — pressure testing of the finished pipe.
• Nondestructive / electric test — where applicable to the product form.
• Radiographic examination — a defining feature of A358's Class system; for A312 it applies where specified.
• Guided-bend weld test — where applicable to welded pipe.
• Heat treatment — the required delivery condition.
• Supplementary requirements — added by the buyer when the application or code requires.

Distinguish three levels of applicability: what is a standard requirement, what is conditional on Class or product form, and what is a buyer's supplementary requirement. For example, radiographic examination is a Class-driven requirement for A358, but is not a default requirement for every A312 order.

8. MTC, PMI and Quality Acceptance

Verification of either standard relies on two complementary tools:

• MTC (Mill Test Certificate) — the documentary record showing the heat number, grade, UNS, chemical composition, mechanical properties, heat treatment, manufacturing method, standard designation, and test results. It is the traceability evidence for the material.
• PMI (Positive Material Identification) — on-site verification, typically by handheld XRF or OES, confirming the material's major alloying elements match the expected grade.

On receipt, confirm that the pipe marking and heat number match the MTC, and that the manufacturing method and standard designation on the certificate match the order. PMI is not a default requirement of A312 or A358; it is a supplementary receiving verification. And PMI does not replace the MTC — it confirms identity, not the full certified test record.

9. When to Choose ASTM A312

A312 is typically the correct choice when:

• A seamless pipe is required by the design or code.
• Standard welded austenitic stainless steel pipe is needed.
• Heavily cold worked pipe is required for elevated strength.
• The application is a conventional piping system where A312 is the governing product specification.

10. When to Choose ASTM A358

A358 is typically the correct choice when:

• Electric-fusion-welded (EFW) construction is specified.
• Larger-diameter or fabricated welded pipe is required, where EFW and a defined Class system are appropriate.
• The project requires a defined radiographic examination level through the Class system.
• The application involves high-temperature or corrosive service with code or specification requirements that point to A358.

Do not assume one standard is inherently more corrosion-resistant than the other. For the same stainless steel grade, corrosion resistance is determined primarily by the alloy, the environment, and the manufacturing/heat-treatment condition — not by the standard designation. The standard governs the product form and inspection, not the alloy's corrosion behavior.

11. How to Write the Purchase Specification

The following are example purchase specifications (not ASTM standard text):

A complete specification states: grade + UNS, ASTM standard and edition, manufacturing method (seamless / welded / EFW), NPS and wall thickness or schedule, length, heat treatment, testing, MTC type, and any supplementary inspection. For A358, the Class must be included.

12. Common Purchasing Mistakes

• Treating A312 as seamless-only. A312 also covers welded and heavily cold worked pipe.
• Treating A358 as a generic ERW standard. A358 is specifically electric-fusion-welded pipe.
• Treating A358 Class as a simple quality ranking. The Class defines weld configuration and radiography, not a material quality grade.
• Not specifying the manufacturing method. Seamless and welded pipe are not interchangeable by default.
• Not specifying the A358 Class. The Class controls weld and inspection requirements and must be stated.
• Specifying NPS without wall thickness or schedule. NPS alone does not define the pipe.
• Confusing the standard with the material grade. A312/A358 are product standards; the grade (e.g., 316L) is specified separately.
• Confusing MTC with PMI. They serve different purposes and are not interchangeable.
• Not confirming radiography or testing requirements. Radiography is Class-driven for A358 and must be defined.
• Referencing an outdated or inapplicable standard edition. Confirm the current edition or the contract-specified edition.

13. Buyer's Selection Checklist

Use this sequence to choose and specify correctly:

• Product form (standard pipe vs. larger-diameter fabricated welded pipe)
• Manufacturing method (seamless / welded / heavily cold worked / EFW)
• Grade and UNS designation
• NPS, outside diameter, wall thickness or schedule, and length
• Standard — ASTM A312/A312M or ASTM A358/A358M (with edition)
• For A358 — the Class (1–5)
• Testing and radiography requirements
• Heat treatment / delivery condition
• MTC type (e.g., EN 10204 Type 3.1)
• Supplementary inspection (PMI, third-party, etc.) where required

Frequently Asked Questions

Q1: What is the difference between ASTM A312 and A358?
ASTM A312 covers seamless, welded, and heavily cold worked austenitic stainless steel pipe. ASTM A358 covers electric-fusion-welded (EFW) austenitic stainless steel pipe, with a Class system defining weld configuration and radiographic examination.

Q2: Is ASTM A312 seamless or welded?
Both. A312 covers seamless, welded, and heavily cold worked austenitic stainless steel pipe. The manufacturing method must be stated on the order.

Q3: What is ASTM A358 pipe?
ASTM A358 pipe is electric-fusion-welded (EFW) austenitic chromium-nickel stainless steel pipe for high-temperature service and general applications, supplied with a Class designation for weld and radiography requirements.

Q4: What is the difference between A312 welded and A358 pipe?
Both can be welded pipe, but A358 is specifically electric-fusion-welded with a Class system for weld configuration and radiography. A312 welded pipe is governed by A312's own requirements. The standards are not interchangeable.

Q5: What are A358 Class 1 to Class 5?
The A358 Classes define the weld configuration (single-weld vs. double-weld) and the required radiographic examination (full, spot, or reduced/as agreed). They are manufacturing and inspection requirements, not a material quality ranking. Confirm the exact Class definitions against the current standard.

Q6: Which standard is better for high-temperature service?
Neither standard is inherently "better." A358 is frequently specified for electric-fusion-welded, larger-diameter pipe in high-temperature or corrosive service where a defined Class and radiography level are required. The correct choice depends on the product form, manufacturing method, and code requirements — not a simple ranking.

Q7: Does A358 require radiography?
Radiographic examination is defined by the A358 Class. Some Classes require full (100%) radiography, others spot or reduced examination. The applicable requirement depends on the specified Class.

Q8: What information should be included when ordering A312 pipe?
Grade and UNS, standard and edition, manufacturing method (seamless/welded/heavily cold worked), NPS and wall thickness or schedule, length, heat treatment, testing, and MTC type.

Q9: What information should be included when ordering A358 pipe?
Grade and UNS, standard and edition, electric-fusion-welded construction, Class (1–5), NPS and wall thickness or schedule, length, heat treatment, radiography and testing requirements, and MTC type.

Q10: Can A312 and A358 pipe be used interchangeably?
No. They govern different product forms and manufacturing methods with different inspection requirements. The correct standard must be selected based on the product form, manufacturing method, and the applicable code or specification.